Interfacial strength and chemistry of additive-free silicon nitride ceramics brazed with aluminium

Two kinds of additive-free silicon nitride ceramics were brazed with aluminium; one was with as-ground faying surfaces and the other was with faying surfaces heat-treated at 1073K for 1.8 ksec in air. The heat-treatment of the silicon nitride ceramics formed a silicon oxynitride layer on the faying surfaces and increased the brazing strength of the joints. A silica-alumina non-crystalline layer and a β′-sialon layer were formed successively from the aluminium side at the interface of the joints. The heat-treatment which made the former layer thicker is a necessary process in making reliable, strong brazed joints.     

Factors influencing the crystallization of ultrafine plasma-synthesized silicon nitride as a single powder and in composite SiC-Si3N4 powder

A study of various parameters affecting the crystallization process of amorphous silicon nitride produced by plasma gas-phase reaction was undertaken to determine the conditions under which whiskers are formed. This process is influenced by the ammonium chloride content of the starting powder and the presence of nitrogen in the furnace atmosphere. This last parameter is also influential on the / phase ratio, along with other factors like the silica content, temperature and duration of the thermal treatment. Heat treatment at 1500°C for 30 min under argon produced well-defined -Si₃N₄ crystals with a hexagonal cross section, a mean length around 0.8 m, and no sign of agglomeration. Under the same conditions, crystallization of silicon nitride in SiC-Si₃N₄ composite did not give crystals, but Si₃N₄ whiskers. Therefore silicon carbide plays a major role in their formation.     

Reaction sintered silicon nitride

The strength, composition and structure of reaction sintered silicon nitride formed in the presence of oxygen or water vapour have been investigated. It is shown that chemical contamination of the nitriding gas by oxygen and water vapour to relatively high concentrations is not the cause of abnormally low strength silicon nitride. The coefficient of variation in strength and apparent crystallite size for a particular batch of samples are related to the water vapour concentration in the nitriding gas. The formation of - and -silicon nitride is discussed and it is suggested that compositional variations in -silicon nitride must be small.     

Impact damage behaviour of CVD-coated silicon nitride for gas turbines

Impact tests were conducted on the silicon nitride substrates coated with Si₃N₄ and SiC by chemical vapour deposition (CVD). For both 100- and 200-m-thick Si₃N₄-coated silicon nitride, Hertzian crack extension was reduced by debonding at the interface. Although Hertzian crack extension was not reduced for 100-m-thick SiC-coated silicon nitride, it was reduced for 200-m-thick SiC-coated silicon nitride. Theoretical calculations suggest that debonding at the interface consumed the fracture energy of Hertzian crack extension in the case of Si₃N₄ coatings, but it was observed that Hertzian cracks were not arrested at the interface.     

The development of strength in reaction sintered silicon nitride

The development of strength in reaction sintered silicon nitride has been investigated by determining the elastic moduli, fracture mechanics parameters, strengths and critical defect sizes of silicon compacts reacted to various degrees of conversion using static or flowing nitrogen. The relationship between each property and the nitrided density is shown to be independent of the green silicon compact density but is influenced by the nitriding conditions employed. Young's moduli, rigidity moduli and strengths vary linearly with the nitrided density. After an initial period when increases may occur, the critical defect sizes in both static and flow materials decrease continuously with increasing nitrided density, although at any particular density they are larger in material produced under flow conditions. A model is suggested for the development of the structure of reaction sintered silicon nitride involving the development of a continuous silicon nitride network within the pore space of the original silicon compact. The experimental data are discussed in terms of the proportion of silicon nitride which contributes effectively to the continuous network.     

Processing aspects of glass-nicalon fibre and interconnected porous aluminium nitride ceramic and glass composites

Glass matrix-fibre and glass infiltrated ceramic composites with interconnected phases have been shown to have the potential for displaying optimum thermal conductivity and dielectric constant at 1 MHz making them useful as substrates for electronic packaging. Ceramic (Nicalon and silicon carbide grade (SCS)) fibre-borosilicate glass composites were fabricated using tape casting processes combined with pressure and pressureless sintering techniques. Experiments were also conducted to process AIN ceramics with interconnected porous channels which were then hot infiltrated with borosilicate glass. Results of optical characterization of the composites indicate that infiltration of Nicalon cloth with glass is achieved by hot pressing, while the tape casting and lamination approach followed by sintering is useful for fabricating composites of glass and Nicalon tows. The sintered aluminium nitride ceramics are comprised of 28% (volume fraction) interconnected pores. Hot infiltration yielded 100 m penetration of borosilicate glass into the pores of the nitride ceramic. The paper discusses the various scientific aspects involved in processing the glass-fibre and porous AIN composites containing 3-d interconnected pores. Results of the microstructural characterization of these composites are discussed particularly in regards to the desired microstructure essential for these composites to be useful as substrates in electronic packaging.     

Mechanical properties of alkoxy-derived cordierite ceramics

Mechanical properties of cordierite ceramics prepared by the controlled hydrolysis and following polycondensation of aluminium, magnesium and silicon alkoxides were investigated in great detail. Flexural strengths of- and-cordierite ceramics are about 60 and 100 MPa, respectively. The flexural strengths of these ceramics are mainly influenced by cracks arising from thermal mismatch between - and -cordierite precipitated during sintering. High fracture toughness of-cordierite ceramics prepared by this method is ascribed to the fine microstructure of the ceramics. The high-temperature flexural strength of-cordierite ceramics is little reduced below 1000° C because of high purity of the ceramics.[/p]     

The nature of sialon joints between silicon nitride based bodies

Quite strong joints between silicon nitride based bodies have been made by incorporating a layer of aluminium and oxides between the bodies and heating in a nitriding atmosphere. The joints are resistant to thermal shock and maintain their strength at 1200° C. Microscopic, DTA and X-ray diffraction studies indicated that sialon phases are present in the joints, and that the bonding reaction involves the reduction of Si₃N₄ by aluminium and the subsequent renitriding of the resultant silicon, as well as the simultaneous nitriding of a portion of the aluminium. Transmission electron microscopy of a joint between hot pressed and reaction bonded silicon nitrides showed that 15R aluminium nitride polytype sialon was present on the reaction bonded side of the joint and ß-sialon on the hot pressed side.     

Reinforcement coatings and interfaces in aluminium metal matrix composites

The interface between the matrix and reinforcement plays a crucial role in determining the properties of metal matrix composites (MMC). Surface treatments and coating of the reinforcement are some of the important techniques by which the interfacial properties can be improved. This review reports the state of art knowledge available on the surface treatments and coating work carried out on reinforcements such as carbon/graphite, silicon carbide (SiC) and alumina (Al2O3) and their effects on the interface, structure and properties of aluminium alloy matrix composites.The metallic coatings improved the wettability of reinforcement but at the same time changed the matrix alloy composition by alloying with the matrix. Ceramic coatings reduce the interfacial reaction by acting as a diffusion barrier between the reinforcement and the matrix. Multilayer coatings have multifunctions, such as wetting agent, diffusion barrier and releaser of thermal residual stress. The roles of reinforcement coating as a means of in situ hybridising and in situ alloying are described.     

Sialon formation by Si+ and N2 + ion implantation into sapphire

Single-crystal alumina was implanted firstly with 400 keV Si⁺ and subsequently with N₂ ⁺ ions and then annealed at 1673 K in an No atmosphre. The implanted layers were characterized by means of X-ray diffraction, Rutherford backscattering-channelling of 2 MeV He⁺ ions, and the resonance nuclear reaction¹⁵N(p,)¹²C. The annealing of sapphire implanted at ambient temperature resulted in the formation of-sialon, a solid solution of-silicon nitride and alumina in the subsurface layer, while implantation at 100 K resulted in the formation of aluminium oxynitride in the surface layer. In the latter case, the implanted silicon atoms were believed not to react vxi1h the implanted nitrogen atoms but with the substrate oxygen atoms. These crystalline precipitates were found to have epitaxial relations with the sapphire substrate.     

Bond strength and interfacial structure of silicon nitride joints brazed with aluminium-silicon and aluminium-magnesium alloys

From the results of the bending strength and Weibull modulus of the joints of silicon nitride ceramics brazed using aluminium-silicon and aluminium-magnesium alloy filler metals at a temperature of 1073 K for 0.9 ksec in a vacuum of 1.3 × 10^–3 Pa, silicon, especially, present in a small amount in the filler metals, was found to be effective in improving the bond strength, while magnesium in the filler metals was harmful to the joining. This can result in the formation of a thick stable alumina layer on the surfaces of the filler metals containing magnesium during brazing which prevented contact of the filler metals with the silicon nitride ceramics.     

Brazing parameters determine the degradation and mechanical behaviour of alumina/titanium brazed joints

The main aims of the present study are simultaneously to relate the brazing parameters with the correspondent interfacial microstructure, the resultant mechanical properties and the electrochemical degradation behaviour of commercially pure titanium/alumina brazed joints. A filler metal on the Ag-26.5Cu-3Ti system has been used. Three different brazing temperatures (850, 900 and 950°C) and three holding times (0.3, 1.2, 2.4 ks) were tested, in order to understand the influence of each combination of brazing temperature holding times, over the final microstructure and properties of the joints. The mechanical properties of the M/C joints were assessed on the basis of bond strength tests carried out using a shear solicitation scheme. The fracture surfaces were studied morphologically and structurally using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction analysis (XRD). The degradation behaviour of the M/C joints was assessed by means of electrochemical techniques. It was found that a brazing temperature of 850°C and a holding time of 2.4 ks, produces the best results in terms of bond strength, 130 ± 16 MPa. The mechanical properties obtained could be explained on the basis of the different compounds identified on the fracture surfaces by XRD. On the other hand, a brazing temperature of 950°C produces the best results in terms of corrosion rates (lower corrosion current), 3.44 ± 0.73 A cm^–2. However, the joints produced at 850°C using a holding time of 1.2 ks present the best compromise between mechanical properties and degradation behaviour, 122 ± 12 MPa and 7.59 ± 1.47 A cm^–2 respectively. The role of Ti diffusion from the metallic Ti to the Al₂O₃ surface is fundamental in terms of the final value achieved for the M/C bond strength. On the contrary, the Ti distribution along the brazed interface does not seem to play any relevant role in the M/C joints electrochemical performance.     

The preparation of alumina fibre by sol-gel processing

Attempts have been made to prepare alumina fibre from the colloidal sol and polymerized alkoxides. The aluminium chloride or aluminium nitrate systems were found to be potential methods for producing continuous alumina fibre: the aluminium nitrate system had a better sintering behaviour than the aluminium chloride system. The aluminium isopropoxide system, however, was unsuitable for preparing alumina fibre but was suitable for the preparation of monoliths, membranes, powders, and multicomponent ceramics. The thermal changes of these precursors were studied by transmission electron microscopy, Fourier-transform infrared spectroscopy and X-ray diffraction. The results demonstrated the different routes of phase transformation as the temperature increases. The aluminium chloride system exhibits two routes for phase transformation: (a) boehmite -Al₂O₃, and (b) gibbsite -Al₂O₃.     

The influence of surface roughness and pre-oxidation state on the wettability of aluminium nitride by commercial brazes

The effects of roughness (Ra = 0.17–0.20 m) and of pre-oxidation of the AlN ceramic surface on its wettability and contact interaction with commercial brazing alloys CB4 and CB5 of Ag-Cu-Ti composition have been studied. Wettability has been determined by the sessile drop method at three holding temperatures (810, 900 and 950°C). Particularities of the interface interaction have been identified by microprobe analysis for pre-oxidized samples. Experimental data are compared with data for samples polished to Ra = 0.02–0.03 m not subjected to pre-oxidation. The results show that, for the systems under study, surface roughness does not influence the contact angle value significantly. Pre-oxidation of the AlN in air at 1250°C, however, tends to reduce wettability as a result of the replacement of braze-aluminium nitride interaction by braze-surface aluminium oxide interaction.     

Microstructure and mechanical properties of transient liquid phase bonds between NiAl and a Nickel-Base superalloy

An investigation of microstructural development in transient liquid phase bonds between the B2 type intermetallic compound NiAl and a nickel-base superalloy MM–247 is presented in this paper. The bonds discussed in the paper employed pure copper interlayers. Based on edge-on transmission electron microscopy investigations, the paper examines both microstructural development at the bond-line and the influence of bonding on the adjacent substrates. The paper considers the epitaxial growth of the B2 type (nominally NiAl) phase into the joint and the formation of non-epitaxial -phase layers. The paper also examines the formation of second-phases, including the L1₂ type -phase (nominally Ni₃(Al, Ti)), MX type carbides, -phase intermetallics and elemental chromium and tungsten.Bond-line and adjacent substrate microstructures for the NiAl/Cu/MM–247 bonds are correlated with joint mechanical properties determined by room-temperature shear testing. The paper compares the microstructure and mechanical properties of NiAl/Cu/MM–247 bonds with those of NiAl/Cu/Ni joints.     

A transmission electron microscope study of a cubic boron nitride-based compact material with AIN and AIB2 binder phases

The microstructure of a commercial cubic boron nitride compact, with aluminium nitride and aluminium diboride binder phases, has been characterized by transmission electron micro-scopy. Within individual grains significant plastic deformation is observed in the form of dis-locations and microtvvin lamellae. There is no evidence of recrystallization. Grains have been forced into direct contact, but there is no evidence of new growth of cubic boron nitride between the grains. Some grains have obviously broken or shattered during manufacture. Aluminium nitride is present mostly as an oriented rind around the cubic boron nitride grains where they are not in direct contact, while aluminium diboride forms a largely continuous network in the channels between them. Our observations suggest that the two aluminium compounds are responsible for most of the bonding in the compact. The orientation relation-ships between the cubic boron nitride and As aluminium nitride rind are, on (1 1 1) facets of BN, BN(1 1 1) AIN(0 0 0 1) and BN[1 1 0] AIN[1 1 – 0], and on (0 0 1) facets of BN, BN(0 0 1) AIN(0 0 0 1) and BN[1 1 0] AIN[l 1 ¨ 0]. The aluminium diboride is mostly unoriented. The results are discussed in the context of previous work on diamond-based compact materials.     

Damage-tolerant laminated composites in thermal shock

Alumina is very susceptible to thermal shock which often leads to catastrophic failure. The addition of tape-cast nickel layers to laminated alumina can increase its tolerance to damage induced during thermal shock. The fracture behaviour after thermal shock of laminated composites showed non-catastrophic failure during loading at room temperature. The retained strength of the laminates was determined for a wide range of quenching-temperature differences (T= 150–1200C). The retained strength and critical quenching-temperature difference, T _c, of the laminated composites were a significant improvement over the values for the respective monolithic alumina. This improvement in behaviour can be related to the compressive residual stress in the alumina layers and ductile-layer crack blunting.     

α-β phase transformation of silicon nitride — computer simulation

The simulation model of - silicon nitride phase transformation was developed for the case when the crystallization of the phase is the rate controlling step. The results gained on the base of the present model indicate that the temperature and total free surface area of silicon nitride phase present in the firing body limit the rate of transformation and total amount of transformed silicon nitride phase. These results are acceptable from the point of view of experimental experience and support the applicability of the presented model.     

Microstructure and mechanical properties of reaction-formed joints in reaction-bonded silicon carbide ceramics

A reaction-bonded silicon carbide (RB-SiC) ceramic material (Carborundum's Cerastar RB-SiC) has been joined using a reaction f rming approach. Microstructure and mechanical properties of three types of reaction-formed joints (350 m, 50–55 m, and 20–25 m thick) have been evaluated. Thick (350 m) joints consist mainly of silicon with a small amount of silicon carbide. The flexural strength of thick joints is about 44±2 MPa, and fracture always occurs at the joints. The microscopic examination of fracture surfaces of specimens with thick joints tested at room temperature revealed the failure mode to be typically brittle. Thin joints (<50–55 m) consist of silicon carbide and silicon phases. The room and high temperature flexural strengths of thin (<50–55 m) reaction-formed joints have been found to be at least equal to that of the bulk Cerastar RB-SiC materials because the flexure bars fracture away from the joint regions. In this case, the fracture origins appear to be inhomogeneities inside the parent material. This was always found to be the case for thin joints tested at temperatures up to 1350°C in air. This observation suggests that the strength of Cerastar RB-SiC material containing a thin joint is not limited by the joint strength but by the strength of the bulk (parent) materials.     

Mechanical properties of monolithic AIN and SiCw/AIN composites

Flexural, tensile, and high cycle fatigue test data are presented for pressureless sintered aluminium nitride (AIN) and hot-pressed aluminium nitride reinforced with silicon carbide whiskers (SiC_w/AIN). Tests were conducted at ambient temperature. The SiC_w/AIN composites consisting of 30wt% SiC_w produced significant increases in flexural strength, tensile strength, and tensile fatigue strength compared to monolithic AIN. Increases were nearly double in all cases. Corresponding strain-to-failures measured in tensile tests increased from 0.04% in monolithic AIN to 0.10% in the SiC_w reinforced composite. Fracture surfaces showed evidence of whisker-toughening mechanisms due to additions of SiC_w whiskers. High-cycle fatigue results indicated that both materials have the ability to sustain higher stress levels in the cyclic tests compared to the tensile experiments. The improved performance under cyclic testing is explained in terms of strain-rate effects. The times at or near peak stress are considerably less under high-cycle fatigue testing (20 Hz) compared to tensile tests (strain rate = 0.5%min^–1).